1. Field of Invention
The present invention relates to radiation film dosimetry, and more particularly to methods and devices for removing effects of variations in film processing and chemistry on radiation dose calibrations.
2. Discussion
An important use of radiotherapy is the destruction of tumor cells. In the case of ionizing radiation, tumor destruction depends on the xe2x80x9cabsorbed dosexe2x80x9d or the amount of energy deposited within a tissue mass. Radiation physicists normally express the absorbed dose in cGy units or centigray. One cGy equals 0.01 J/kg.
Radiation dosimetry generally describes methods to measure or predict the absorbed dose in various tissues of a patient undergoing radiotherapy. Accuracy in predicting and measuring absorbed dose is key to effective treatment and prevention of complications due to over or under exposure to radiation. Although many methods exist for measuring and predicting absorbed dose, radiation film dosimetry is often used because of its excellent spatial resolution (better than 1 mm) and its ability to measure dose distribution. In addition, film dosimetry is an integrative techniquexe2x80x94the measured dose distribution represents the accumulated dose in an irradiated mediumxe2x80x94which is necessary for undertaking dosimetry of dynamic treatment methods such as dynamic wedges, arc, and rotation therapy.
Radiation film dosimetry depends on the phenomenon that photographic or radiographic film will darken when exposed to ionizing radiation. The degree of darkening depends on the amount of radiation absorbed by the energy sensitive layer on the film, and can be quantified in terms of the film""s optical density. One may calculate the optical density, OD, from the expression:                               OD          =                                    log              10                        ⁡                          (                                                Φ                  0                                Φ                            )                                      ,                    I      
where "PHgr"0 and "PHgr" represent, respectively, the intensity of light (photons) striking and passing through the film. Note, however, that all of the methods disclosed below use the term optical density more generally to describe any quantity that provides a measure of film opacity.
In typical film dosimetry, a technician makes one or more calibration films by irradiating unique areas of the films with different radiation dose levels (cGy) using a linear accelerator. Next, the technician develops the calibration films and scans them with a film digitizer, which converts each of the films to an array of pixels having values representing the optical density at each point on a particular calibration film. Knowing the radiation dose levels of the exposed areas of the films, a radiation physicist, usually with the aid of specialized computer software, develops a calibration or HandD curve, which relates film optical density to radiation dose.
Armed with the HandD curve, the radiation physicist can quantify beam characteristics of the linear accelerator through subsequent exposure, development, and OD measurements of radiographic films. For example, as part of a treatment plan or quality assurance procedure, the radiation physicist can use film dosimetry to generate depth dose profiles, isodose and isodensity contours, and cross section profiles. In addition, the physicist can use film dosimetry to perform flatness and symmetry analyses, and to carry out field width calculations, among others. Usually, the physicist uses computer software that automatically calculates and displays beam characteristics from scanned and digitized radiographic films. Useful software for generating the HandD curve and for analyzing radiotherapy beam characteristics includes RIT113 FILM DOSIMETRY SYSTEM, which is available from Radiological Imaging Technology.
Film dosimetry offers many advantages over competing methods, but its use is often limited by its variabilityxe2x80x94a film exposed to the same dose of ionizing radiation as an earlier film may exhibit a significantly different optical density. Much of the variability in film dosimetry can be traced to film processing. For example, in a commercial film processor a temperature change as small as one degree Celsius can cause significant changes in optical density. Other film processing variables, such as the age and strength of chemicals in the film processor, and the number and type of films processed can contribute to changes in optical density. In addition, it is known that films from two different manufacturing lots can exhibit significantly different optical density following exposure to the same dose of radiation.
Three methods have been used to circumvent film-processing variations, but each suffers serious drawbacks. In the first method, a fresh set of calibration films are exposed and developed along with each group of test films that are associated with a separate treatment plan or QA procedure. Thus, for each set of test films, the first method requires that a technician expose an additional ten to twenty five calibration films, which improves the accuracy of film dosimetry but consumes considerable resources. In the second method, the dose at any point on a given film image is expressed as a percentage or fraction of the maximum dose or optical density of the film image. Although the second method, which is known as xe2x80x9crelative dosimetry,xe2x80x9d can correct gross variations in film processing, it is only useful in applications that do not require an absolute measurement of dose.
The third method is similar to the first method, but relies on the observation that despite differences in film processing conditions and film chemistry, most HandD curves exhibit similar shape. Like the first method, a set of calibration films are exposed and developed along with each group of test films. But instead of exposing a full set of calibration films, the technician exposes only two films, which bracket the dose range of the test films. The two filmsxe2x80x94a low optical density film and a high optical density filmxe2x80x94represent xe2x80x9cendpointsxe2x80x9d of the HandD curve. Since most HandD curves have similar shape, one uses the two endpoints to adjust the original HandD curve. In other words, the method approximates the original HandD curve with a mathematical function, and uses the endpoints to adjust the original HandD curve fit. Although the third method appears promising, the radiation therapy community has failed to adopt it. Recent analyses indicate that xe2x80x9cend pointxe2x80x9d calibration corrections may lead to significant error because film-processing variations and film chemistry differences can affect the central region of the HandD curve more than the endpoints. In addition, the method appears strongly dependent on exposure (operator) technique.
The present invention endeavors to overcome, or at least minimize, one or more of the problems described above.
The present invention provides a fast and accurate method of correcting errors in film dosimetry measurements arising from temporal changes in radiographic film processing and manufacturing. The method includes irradiating separate and distinct areas of one or more calibration films with different radiation dose levels to obtain a first calibration that comprises data pairs of optical density (or a quantity related to optical density, such as light intensity) and radiation dose. The method also includes exposing a non-irradiated portion of at least one (and typically all) of the calibration films to light from an array of standardized light sources to obtain a first optical density step gradient. During preparation of test films, which are part of a QA procedure or patient treatment plan, the method provides for exposing a non-irradiated portion of the test film (or films) to light from the array of standardized light sources to obtain a second optical density step gradient. Since the characteristics of each of the standardized light sources change little from one exposure to the next, any deviations in the optical density step gradient among different films result, presumably, from changes in film processing or differences in film chemistry. Hence, the method includes modifying either the first calibration or the optical density values of the test films based on differences between the first and second optical density step gradients.
Unlike conventional methods, the present invention provides an accurate correction over the full range of optical densities encountered in film dosimetry. In addition, use of the inventive method should result in considerable cost savings since it substantially reduces the number of calibration films that must be prepared in conventional film dosimetry. Moreover, for about the same expenditure of time and labor associated with relative film dosimetry, the present invention yields absolute measurements of absorbed dose.